Mechanisms for transporting or tensioning web material used in printers, copiers, plotters for engineering or technical drawings, document scanners, facsimile machines, chart recorders, and the like, typically include one or more rollers that are engageable with spools of the web material. Such mechanisms are usually complex and include several interacting shafts, each of which has one or more rollers mounted thereto. Such mechanisms usually require extreme accuracy, which is difficult to obtain at least for an extended period of time due to, among other things, the force interactions between shafts and the large swings in tensile stress imposed on the web material during web transporting and tensioning. If the mechanism is not able to account for force interactions and the large changes in tensile stress, it often will result in wrinkling or possibly fracturing of the web material. Thus, the force interactions and large swings in tensile stress often impair system performance or cause outright system failure.
Although the changes in tensile stress imposed on the web material can be caused by several factors, one common factor is the inability of the rollers to suitably accommodate spools not meeting precise inner diameter requirements. A secure engagement between the roller and spool is necessary for the mechanism to achieve high precision and performance. Thus, unless the spools are manufactured within very tight tolerance, the rollers prevent the mechanisms from achieving high precision and performance.
Attempts have been made to address the problem of rollers not being tolerant of the inner diameters of spools. For example, it has been known to attach a flexible piece of metal to the rollers to enable the rollers to engage spools having different inner diameters. The piece of metal is attached by screws to the roller within a recessed area defined by the roller, and extends along a portion of the length of the roller and outward from the roller. The piece of metal may include a deflected portion at its leading edge for engaging the inner diameter of the spool.
Although such a flexible metal piece enables the rollers to accommodate a range of different inner diameters, it does have drawbacks. Most notably, the flexible metal piece prevents radial compliance between the inner diameter of the spool and the roller since the flexible metal piece simply forces a portion of the inner diameter of the spool outward. As a result, the flexible metal piece can cause "barrel wobble," which can affect the performance of the mechanism. Another drawback is that it is relatively time consuming to attach the flexible metal pieces to the rollers.
It is an object of the present invention to provide a spring finger assembly for a mechanism for transporting or tensioning web material or other thin membrane material that has a relatively high tolerance for variation of the inner diameter of a spool of thin membrane material.
It is a further object of the present invention to provide such a spring finger assembly for providing radial compliance between a roller and shaft of the mechanism and the spool of thin membrane material.
It is still a further object of the present invention to provide such a spring finger assembly that can easily be engaged with the roller.